Dating as part of a Building Survey
If you have a historic roof, likelihood is, its pretty historic below! well that's a great place to start when trying to speculate about the origins of your property, and yes it is largely speculation unless you're willing to invest in a more detailed research. whilst Internal joinery, plasterwork and other features are often a great indication as to a buildings origins or evolution, they're of often added later - even that date stone above the door can be misleading and can be indications of refurbishment or redevelopment.
Even though we're talking in centuries rather than decades, its fascinating to be able to 'speculate' that the house is late medieval rather than seventeenth century. Below shows a very early roof structure, possibly 15th or C16th century with later 17th century adaptations to accommodate a new plastered ceiling.
Cut marks (indicating 75 degree pit sawn timber of 17th or 18th century or 45 degree cuts of a tressel cut timbers of the medieval period), the presence of clasped or entrenched purlins and ridge beams and, as below a nicely formed spliced chamferred tie can indicate the difference between a roof thought to be 17th century and an earlier late medieval or 16th century and potentially the presence of a single storey 'hall' style structure.
Of course we're usually faced with a rich tapestry of features and details, showing a building (often until the 20th C) constantly in change